Please refer to FIG. 1, it shows schematically a front view of a keyboard of personal digital assistant in prior art. As usual, a qwerty keyboard 4 of a personal digital assistant 2 (PDA) has about thirty-five to forty isolated buttons, and each of the buttons 402 is corresponding to a letter 6 in English alphabet or Chinese alphabet, such like A, B, C, or   respectively. The arrangement of the qwerty keyboard 4 having 35-40 keys can also be the same as that of a conventional 101-key keyboard. Some specific buttons (i.e., the dual-function buttons) of the qwerty keyboard 4 may be functioned at the same button to represent both a letter button 402 and a number button 404, due to the limited surface area of the PDA 2. It is usually inconvenient to switch these dual-function buttons between the letter functions and the number functions, while users operate the PDA 2.
The switched operation on the qwerty keyboard is not quite easy as it presents, especially when it is applied to the mobile phone which has only twelve buttons.
Some qwerty keyboards have isolated number buttons arranged in a top row of the keyboard or around a side of the PDA 2. However, since user is costumed to performing a thumb-inputting operation on the qwerty keyboard, the above-mentioned qwerty keyboard is still cumbersome for the user to introduce the number function therefrom. Definitely, it is inevitable to expect operation or input errors from operating dual-function buttons in the qwerty keyboard.
Therefore, an improved input device without trading off the user comfort is definitely popular to the skilled person in the art.